This invention relates to a key top element for a switch suitable for use for a mobile communication device such as a portable telephone, a domestic telephone, an electronic pocket notebook, a measuring instrument, a vehicle-mounted switch, a remote controller, a data input unit for a computer or a personal computer, a switch unit or the like and an element for a push button switch (hereinafter referred to as "push button switch element") including such a key top element and a method for manufacturing the same, and more particularly to a key top element suitable for use for not only a display section for displaying a character, a symbol, a figure or the like while exhibiting enhanced design properties, visibility and durability but a back-lighted push button switch and a push button switch element including such a key top element.
In general, in a push button switch for a unit such as a mobile communication device like a portable telephone, an electronic pocket notebook, a measuring instrument, a remote controller or the like, a push button switch element or a cover element including a key top element is received in a casing of the unit while being mounted on a circuit board, to thereby provide a push button switch for operating a circuit on the circuit board. The key top element used for such a push button switch element is made of a plastic material and has a character, a numeral, a symbol or the like printed thereon as required. In recent years, a key top member of the key top element is proposed which is formed of a transparent resin material and has a character, a numeral, a code, a symbol or the like printed on a rear surface thereof in the form of a printed layer, resulting in the printed layer being protected with the key top member and the key top element exhibiting a high-quality feeling. Also, the key top element is often realized in the form of a back-lighted push button switch using an LED or the like.
Such a key top member for a push button switch is formed of a transparent thermoplastic resin material such as polyester, polycarbonate, acrylic resin, styrene or the like or a hardening or thermosetting resin such as silicone, urethane, unsaturated polyester, vinyl ester, acrylic resin by injection molding, compression molding, cast molding, transfer molding or the like. Then, the key top member thus prepared is formed on a rear surface thereof with a symbol by screen printing, pad printing or the like. Also, as required, a plurality of such key top members are adhesively arranged on a cover substrate made of rubber, a conical spring made of polyester or the like and then incorporated in a key board section of a portable telephone or the like.
As described above, in the conventional key top element, printing of a symbol or the like is carried out directly on the rear surface of each of the key top members. Thus, screen printing or pad printing is conventionally employed for this purpose. Unfortunately, screen printing or pad printing requires to prepare a design picture called a block copy depending on a color of a print design and prepare a screen plate or an intaglio using the block copy thus prepared. Also, it requires to use a printing plate of a configuration corresponding to a symbol such as a character, a numeral, a code or the like for every color, so that it is required to carry out the printing by the number of times corresponding to the number of colors to be used. Thus, the prior art renders the printing highly troublesome and causes design properties to be deteriorated, resulting in substantially failing to print a distinct photograph, illustration, picture or the like with increased definition. In addition, the prior art causes the cost for plate making to be increased when diversified small-quantity production takes place and renders a change in design highly troublesome.
Also, the printing is made on the rear surface of each of the transparent key top members, so that an increase in thickness of the key top member causes a printed region on the key top member to be deeply positioned in a unit such as a portable telephone when it is incorporated in the unit. This fails to permit external light to satisfactorily reach the printed region, so that the printed region looks dark and dull when it is observed using reflected light, resulting in being deteriorated in visibility. Such visibility is ensured when the key top member is formed to have a thickness as small as about 1 mm or less. Unfortunately, the prior art fails to provide the key top member of such a small thickness.